Generally, a loop antenna is formed in the shape of a tetragonal loop, a circle loop, or the like and is used in various fields according to a length thereof.
The loop antenna has a characteristically low input resistance. In order to match a 50Ω input resistance of a general antenna, the length of the loop antenna should be taken into account in its design.
According to an impedance curve of a square-shaped loop antenna, an input resistance comes close to 50Ω and an input reactance comes close to 0 only when the length of the loop is near to one wavelength. That is, the loop antenna causes resonances only when it is designed to have the length of one wavelength.
Also, the loop antenna has a radiating pattern which changes according to the length thereof. For instance, the loop antenna radiates electromagnetic waves along a plane direction thereof when the length of the loop antenna is shorter than one wavelength, and along a direction vertical to the plane direction thereof when it is longer than one wavelength. Accordingly, the radiating pattern of the loop antenna can be adjusted by adjusting of the length of the loop antenna.
However, if the radiating pattern is adjusted by forming the length of the loop antenna to be shorter or longer than one wavelength as described above, it is difficult to match the input resistance and the input reactance due to characteristics of the loop antenna. Accordingly, a device to which the antenna is mounted should be equipped with a separate matching circuit for matching the input resistance and the input reactance.
However, if the device is equipped with the separate matching circuit, it requires a space for installing the matching circuit. Also, there is a disadvantage in that if a design of the matching circuit should be changed due to interference with other circuit elements after the matching circuit is mounted to the device, it is not easy to change the design of the matching circuit.
Thus, there is required a new method capable of minimizing the space which the matching circuit occupies thereby reducing the device in size, and easily changing the design of the matching circuit.